Using a kite for wind-driven propulsion of water, land, ice and or snow-craft is highly effective and simple compared to other means of wind-driven propulsion like sails and turbines. A kite can be constructed in such a way that all or most of the pulling force enters the craft in one point, without resulting momentum that can tip the craft over and without the requirements for rigid elements like masts, poles and shafts connected to the craft. Kites are flying remote from the craft, in air layers with stronger winds, and can fly in figures across the sky in order to generate apparent wind, thus increasing the power. Thus, kites are very efficient as means to derive power from the wind.
Traction kites need to be manipulated constantly in order to control the flying trajectory of the kite to generate the required traction, to handle wind-gusts, and to keep the kite from falling to the ground.
For smaller crafts, like buggies, surfboards and dinghies, the kite can be controlled by hand. One common way of manipulating a kite is by means of a 3 point control system, where one centrally placed line bears the main load of the kite, and two control lines are manipulated to sheet in and out the flexible left and right tips of the kite. The centrally place line attaches to a structural part of the craft, or, in case of surf style kiting where the rider stands on a wheeled or sliding board, to a point on a harness worn by the rider, and whereby the two control lines attach to the ends of a control bar. The control bar often features a central hole through which the central load bearing line is slideably lead. In analogy to sheeting a sail on a sail craft, by sliding the bar away from the fixed point on the central load bearing line, the tips of the kite are sheeted out, allowing the wind to pass the kites canopy, while by sliding the bar towards the fixed point on the central load bearing line, the kite catches more wind and will thus power up. By puffing one side of the bar, the kite will sheet in on the side pulled, and out on the other. This will cause the kite to turn around the sheeted inside.
The sliding motion or ‘stroke’ of the bar is limited on one side by the assembly that attaches to the rider's harness, and on the other by either the length of the rider's arms or parts of the control system.
In order to control the kite comfortably and safely, kite control systems all allow for some degree of correction for stronger winds or larger kites, or to adapt the kite to a different riding style. By correcting the length of the load bearing line relative to the control lines, the bar stroke can be matched with the circumstances and style of the rider.
Most kites feature correction or ‘trim’ systems that shorten or lengthen the central load bearing line, either by using cleat or friction based systems. Because in this case only one line needs to be shortened or lengthened, such systems are simple to build. Yet, because it is the load bearing line the rider is trimming, operating requires lots of force. Obviously, such systems have to be built very strong and heavy, while all parts of such trim assemblies are subject to wear. Also, trimming the central load bearing line can only take place on either side of the bar stroke, close to the rider's body or away from the rider at arm's length, neither a good ergonomic location for precisely adjusting of trim controls.
Another major disadvantage of central load bearing line trimming is that rider error, or failure of a cleat or strap system, can result in the central load bearing line assuming its maximum length, which enlarges the projected area of the kite canopy towards the wind. A kite can thus become heavily over-powered and instantly cause extremely dangerous situations for both rider and bystanders.
Yet another disadvantage is that trimming the central load bearing line is mostly done by short pulling motions on that line in a direction away from the kite. By tugging on the central load bearing line, which attaches to the front side of the kite, the kite can suddenly over fly and fall from the zenith. This can result in dangerous situations, varying from line tangles to sudden tightening of lines and uncontrolled looping of the kites.
A smaller disadvantage, but worth mentioning, is the behavior of bar stoppers on center line trimmed kites. Various types of bar stoppers are installed on different kite control systems, mostly to limit the up stroke of the bar. Control systems that trim the center line from the harness attachment point, referred to as ‘below the bar trimming’, will have the stopper moving along with the trimming motion of the line. This implies that the rider needs to re adjust the stopper after trimming the center line.
An alternative to central load hearing line trimming is control line trimming, like described in DE20315464U1 (2003) and US2012018584A1 (2011). Both these publications show methods for guiding the control lines in to the bar, where they are joined, while a single trim line is attached at the joint. The trim line is in turn led out of the bar through an opening and can be pulled or released to set the length of the control lines, and locked by means of a clamping device. By doing so, the left and right control line can be trimmed simultaneously. A disadvantage of such systems is that the trim line can dangle freely from the bar, which is annoying and can be dangerous as the trim line can wrap around the riders hand or tangle with other lines in the control system. Even if means for retaining the line are present as for instance hook and loop patches or a magnet, these will have to be handled by the user, which takes away attention and focus from maneuvering the kite. Another disadvantage of these particular systems is that the pulling direction is perpendicular to the bar or respectively away from the end of the bar. Both pulling movements are very unpractical, and it will be difficult to keep the kite flying a straight line during trimming. Yet another disadvantage is that clamping a line by friction induces wear to both line and clamping device, while slippage can occur at such clamps. Furthermore, it is difficult to see how much control line is pulled in; one would need to judge this by estimating the length of line dangling from the bar. A big disadvantage of friction based clamping devised like cleats is, that they lock tighter and tighter over time, which leads to considerable operating forces. Applying such forces to a kite control bar can induce sudden control error, which is obviously dangerous.
In the early days of kite surfing, some reel bars were manufactured, like for instance U.S. Pat. No. 6,877,697. These mechanisms could be used to trim the lines relative to each other, but their size, complexity and number of moving parts would make them unsuitable for kite surfing. Sand ingress in to a bar is impossible to avoid, and sea water, sea-weed, and all sorts of debris found on most kite beaches as well as snow and ice for snow-kiters will render a too complex mechanism useless in minutes, unless a most simple design is used.
As larger kites need more leverage to be steered, kite bars are used in different sizes. It is not uncommon for a rider to own two or three different length bars. In order to limit the number of bars a riders needs for his quiver of kites, some kite bars offer adjustable length, either by a telescopic part towards the end of the bar or by offering multiple fixing points for knotting the kite control lines. Undoing the usually very tight knots to change the effective length of a bar is hard to do, especially when a kiter has cold and wet fingers. An obvious disadvantage of a telescopic construction is their sensitivity to sand and salt, while another disadvantage is the indirect feel of the bar when the bar ends can rattle a bit.
Kite bars are most commonly made from tubular alloy or glass fiber elements with separate bar-ends, joint with adhesive and rivets. A usual way of building the central section with the central hole is to use an alloy centerpiece with tubular segments attached to either side. These rather complex multi-part assemblies induce a risk of breakage of the bar.
These current bar constructions do not allow for much functional geometry on the inside of the tubular element. Routing lines through the bar and passed the central hole, and adding clamping means is difficult, and the function will be limited as sand, water and or ice will accumulate inside the bar.
It needs to be mentioned that use of propulsive wings and traction kites involves a number of risks, some of which already discussed above. Pilot error as well as material error can cause serious injury and even death as the forces produced by kites can quickly amount to dangerous levels. Designing kites and kites control systems needs to involve very serious failure analysis of every part.